To describe this fluid transmission device using a torque converter for a vehicle as an example, the device comprises a converter mechanism constituted by a pump impeller connected to an engine output shaft via a cover member, a turbine runner disposed opposite the pump impeller and connected to a transmission input shaft, and a stator disposed between the pump impeller and the turbine runner, and is constituted such that the power of the engine output shaft is transmitted to the transmission input shaft via a working oil that flows through a converter chamber formed in the interior of the converter mechanism. A torque converter having a lockup mechanism, in which a lockup mechanism is provided in a lockup chamber surrounded by the cover member and the turbine runner such that power transmission can be performed by directly coupling the engine output shaft and the transmission input shaft, is also widely known. A multiple disc clutch mechanism provided with a plurality of plate-shaped frictional engagement members may be used as the lockup mechanism.
In a typical torque converter having a lockup mechanism, the lockup mechanism is set in a released state at low speed such that power transmission between the two shafts is performed via the converter mechanism, and at high speed the lockup mechanism is set in an engaged state such that the two shafts are directly coupled and power transmission is performed bypassing the converter mechanism. In a transient state between release and engagement, control is performed to cause the lockup mechanism to slide, and therefore power transmission is performed in accordance with the operating conditions of the vehicle, enabling an improvement in fuel economy.
However, when this type of sliding control is performed, the amount of heat generation from the frictional engagement members increases, and this may affect the durability of the lockup mechanism. To counter this effect, a torque converter in which working oil supplied to the converter chamber is led into the lockup chamber so that the lockup mechanism is cooled by the working oil has been proposed in the related art (see Japanese Unexamined Patent Application Publication H6-221403, for example). In this torque converter, a passage is formed to lead the working oil from the converter chamber to the lockup chamber, and the lockup chamber (lockup mechanism) is provided in a passage for discharging the working oil in the converter chamber to the outside.